GC Content Calculator
Calculate GC content percentage of a DNA or RNA sequence. Find the proportion of guanine and cytosine bases, which affects melting temperature, primer design, and sequence stability.
🧬 Molecular📐 GC% = (G+C)/(A+T+G+C) × 100🧬 Biology
Number of G bases
Number of C bases
Number of A bases
Number of T bases (or U)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| GC Content Calculator | — | GC% = (G+C)/(A+T+G+C) × 100 | % |
Step-by-Step Examples
Example 1
Simple sequence
G=300, C=280, A=220, T=200. N=1000 bp.
- GC = (300+280)/1000 = 580/1000 = 58%
- AT = 42%
- Tm ≈ 81.5+0.41×58-675/1000 = 81.5+23.78-0.675 = 104.6°C
✓ GC% = 58%, Tm ≈ 105°C
Example 2
PCR primer check
Primer: GCATGCGTACGATCGATCG. G=5,C=5,A=3,T=5. N=18.
- GC = 10/18 × 100 = 55.6%
- Good primer GC range: 40-60%
- Tm (simplified) = 4(G+C)+2(A+T) = 40+16 = 56°C
✓ GC%=55.6%, Tm≈56°C (good primer)
Example 3
Human genome
Human genome: ~41% GC content.
- GC = 41%, AT = 59%
- Varies by chromosome and region
- CG islands: GC > 55%, associated with gene promoters
✓ Human genome: 41% GC overall
Real-World Applications
PCR Primer Design
GC% 40-60% optimal for primers. High GC: risk of secondary structures. Low GC: poor primer binding.
Phylogenetics
GC content varies between organisms: Mycobacterium (65%), Humans (41%), AT-rich parasites (20%). Reflects mutational bias.
Microbiology
GC content used to classify bacteria before gene sequencing. High GC: Actinobacteria. Low GC: Firmicutes.
Bioinformatics
GC content affects codon usage, RNA secondary structure stability, and nucleosome positioning.
Common Mistakes to Avoid
⚠️
G pairs with C, A pairs with T/U
Three hydrogen bonds in G-C pair vs two in A-T. High GC = higher thermal stability, higher Tm.
⚠️
Chargaff rules: %G=%C and %A=%T
In double-stranded DNA: G=C and A=T (Chargaff's rules). GC% can be calculated from either strand.
⚠️
GC affects more than just Tm
High GC: harder to amplify by PCR (stable secondary structures), different codon usage, different protein amino acid composition.
Frequently Asked Questions
What is GC content? ▾
Percentage of nucleotides that are G or C. Varies from ~20% (some parasites) to >70% (some bacteria) and correlates with genome stability, gene density, and mutation bias.
Why does GC content affect melting temperature? ▾
G-C base pairs have 3 hydrogen bonds (vs 2 for A-T). More energy needed to separate GC-rich strands. Rule: Tm = 81.5 + 0.41(%GC) - 675/N.
What is a CpG island? ▾
Genomic regions with GC > 50% and CpG dinucleotide frequency > 0.6 of expected. Found in 60-70% of mammalian gene promoters. Often unmethylated, enabling gene expression.
How does GC content affect PCR? ▾
Sequences with > 65% GC or < 35% GC are harder to amplify. GC-rich templates form secondary structures. Additives (DMSO, betaine) or touchdown PCR help.
What is GC-skew? ▾
GC-skew = (G-C)/(G+C). Measures asymmetry between G and C in a single strand. Positive on leading strand in many bacteria. Used to find replication origins.